Making waves: Enhancing sustainability and resilience in coastal cities through the incorporation of seawater into urban metabolism.

Water and energy are critical components of urban metabolism. However, climate change-induced water scarcity and elevated temperatures pose a significant threat to the adequate supply of essential human services, including sanitation and space cooling, particularly in coastal cities where over 40% of the population resides. The water-energy nexus of sanitation and space cooling is crucial for promoting sustainability and resilience in coastal cities. For decades, Hong Kong has demonstrated the effectiveness of using seawater for toilet flushing and district cooling to save water and energy, which could serve as a potential solution for other coastal cities worldwide. Seawater is a superior alternative to other sources of toilet flushing water due to its abundant availability, easy detection of cross-contamination, and lower treatment costs. Furthermore, saline wastewater treatment requires fewer materials and energy inputs and produces less sludge. Using seawater for district cooling also saves energy without exacerbating water stress. However, there is a lack of comprehensive insights from Hong Kong on how seawater use can be adopted by other coastal cities to promote sustainable development. A successful introduction of seawater into coastal cities requires a holistic water-energy management framework that provides technical and policy-level guidance. We developed such a framework that follows four sustainability principles, namely customized solutions, efficient resource allocation, comprehensive evaluation, and optimized tradeoffs. These principles are designed into contextualized location analysis, urban spatial analysis, integrated sustainability assessment, and nexus analysis. The results of these analyses can aid decision-making regarding the technical and policy aspects of seawater uses in sanitation and space cooling to maximize the positive impacts on sustainable development. Breaking barriers between sectors and encouraging inter-municipal cooperation between sectors are critical to the successful use of seawater. By adopting this framework and promoting collaboration across different sectors, coastal cities can enhance their sustainability and resilience, providing a better quality of life for their citizens.